1. Field of the Invention
The present invention relates to a metal structure catalyst and a method of preparing the same.
2. Discussion of Related Art
In conventional chemical processes (preparation of hydrogen, hydrodesulfurization, etc.), a packed bed catalyst reactor has been mainly used. Conventionally, a packing tower reactor enabling reaction heat of a high temperature has problems of fundamental disadvantages of a ceramic (alumina, cordierite, etc.)-supported catalyst, including a decrease in catalyst utilization efficiency due to a low heat and mass transfer rate and an increase in reactor volume according thereto.
Xu and Froment explains that, since an actual steam reforming reaction has a catalyst effectiveness factor of approximately 0.03, a mass transfer resistance through a catalyst pore is very high [AIChE J, Jianguo Xu and Gilbert F. Froment, Methane steam reforming, methanation and water-gas shift: I. Intrinsic kinetics, 35, 1989, 97]. In addition, a packed bed catalyst reactor has problems of degradation in reactor performance according to high pressure drop and a channeling of a reactant, and a slow response characteristic according to initial starting time and load fluctuation by a low thermal conductivity of a ceramic catalyst.
To overcome the pressure loss of the conventional packing tower catalyst reactor, a structure composed of a channel was used as a catalyst supporter. In a high temperature endothermic reaction process, for example, a reforming reactor, a metal structure having an excellent heat transfer characteristic, rather than a structure formed of a ceramic material which is vulnerable to a thermal shock, was employed as a catalyst supporter [Korean Patent Application No. 1993-0701567 and 2003-0067042].
A general metal structure has a boundary layer formed on an inner surface of a channel due to characteristics of the long channel including a cell density of approximately 200 to 400 cpi and a ratio (L/D) of a length to a diameter of the channel of approximately 70 to 120, and thus heat and mass transfer is limited and it is difficult to coat uniformly the channel with a catalyst due to a capillary phenomenon.
Generally, types of a metal structure include a metal monolith, mat, form, and mesh. When a metal material is used as a catalyst support, due to physical binding and a difference in coefficients of thermal expansion between a metal and a ceramic carrier, a catalyst or a catalyst-supported carrier is detached from the metal structure at high temperature, and thus durability and activity of the catalyst are degraded.
To ensure thermal shock stability and to enhance an adhesive strength of the catalyst attached to a surface of the metal structure, a prior art relating to the conventional metal monolith structure catalyst has been developed.
In Korean Patent Application No. 2002-0068210, to enhance an adhesive strength between a metal surface and a catalyst, aluminum metal particles were primarily coated on a surface of a metal structure as a protective layer for preventing metal corrosion, and aluminum metal particles serving as carriers were secondarily coated thereon in a porous shape. An inter-layer alloy is formed according to thermal treatment after coating each layer, and thus cracks and detachment are prevented. In addition, a metal-metal oxide layer is formed by oxidation at high temperature. Finally, a monolith-type catalyst module including a metal structure was manufactured by attaching a catalyst to a metal oxide layer by a wash coating method.
In addition, in Korean Patent Application No. 2005-0075362, to improve an adhesive strength between a substrate and a catalyst, the same material as the catalyst or a material having the same surface characteristic as the catalyst was coated at an interface between the substrate and the catalyst as an adhesive layer by performing atomic layer deposition (ALD) or chemical vapor deposition (CVD) on a surface of the substrate. This technique has an advantage in that uniform coating to a desired thickness is possible regardless of the kind and shape of a substrate. However, a metal oxide is formed by repeatedly forming M-OH (M: metal) bonds by a reaction between a hydroxyl group on a surface of a metal and a metal precursor. Due to the limitation to a specific metal precursor capable of forming a M-O-M bond by a reaction with a hydroxyl group, the use of expensive reaction equipment and performance under a vacuum, there is a limitation to ease of utilization of the art and compatibility. For reference, here, the catalyst was wash-coated by being mixed with alumina sol.
In Korean Patent No. 835046 and 670954, a catalyst was carried to a porous catalyst support (metal foam, ceramic foam, metal felt, metal screen, etc.). To enhance an adhesive strength between a metal surface and the catalyst, an interface layer (alumina, alumina+silica, titania) was coated on an oxidized FeCr alloy felt using metal organic chemical vapor deposition (MOCVD). Afterward, a powdery catalyst slurry was prepared and carried by wash-coating, or catalyst coating was performed by directly dipping into a precursor solution of an active metal. Such an intermediate layer has a similar component to a carrier layer, and is actually applied to a carrier of the catalyst, and the catalyst was generally coated by dip coating or wash-coating, chemical vapor deposition, or physical vapor deposition.
Korean Patent No. 696622, related to manufacture of a micro reforming reactor composed of a microchannel, pointed out that it was difficult to uniformly and selectively coat desired parts with conventional coating techniques, and a flow coating method was applied to coat a catalyst only on an inner wall of a microchannel. The flow coating method is a method of flowing a slurry coating solution of a powdery catalyst and injecting air, which may control a thickness of a coating layer depending on a viscosity of the coating solution and a rate of air injection. However, the method of this patent is also not that different from the conventional wash-coating method, and thus has a difficulty in uniform coating of a corner in the microchannel. In addition, when a thin catalyst layer is coated with a small amount of the catalyst due to the limitation on a coated amount according to a thickness of the catalyst layer and the influence of catalyst activity, catalyst performance is low, but when a large amount of catalyst is coated, the thick catalyst layer is detached, and thus catalyst performance may not be activated.
As described above, the conventional arts usually focused on formation of an inter-binding layer to enhance a binding strength between a metal surface and a carrier layer to solve a detachment phenomenon caused by a difference in coefficients of thermal expansion of the metal surface and a ceramic catalyst in order to develop a metal structure catalyst.
However, another problem is deactivation of a structure catalyst which may be caused by reducing an active surface area due to sintering of catalyst particles according to a thermal shock in a high temperature reforming reaction in which steam is actually present or a reaction startup-shutdown process. In addition, at a high gas hourly space velocity (GHSV) providing a large amount of reactants, only a active site exposed on a catalyst surface participates in the reaction, and therefore it is necessary to highly disperse catalyst on a surface of the metal support to develop a high-activity metal structure catalyst. Accordingly, in development of a metal structure catalyst having high activity and high durability, to enhance activity of the catalyst as well as to ensure high durability according to an enhanced binding strength between a surface of the metal support and a carrier, highly dispersed catalyst on a surface of the carrier is required.
Conventionally, in the manufacture of the metal structure catalyst, a method of impregnating a catalyst into a precursor solution of the catalyst to support the catalyst [L. Villegas, F. Masset, N. Guilhaume, ‘Wet impregnation of alumina-washcoated monoliths: Effect of the drying procedure on Ni distribution and on autothermal reforming activity’, Applied Catalysis A: General, 320 (2007) 43-55] and a wash-coating method for coating a catalyst on a metal surface with a slurry solution prepared by mixing a powdery catalyst prepared by previously supporting a catalyst to a carrier with alumina sol [J. H. Ryu, K.-Y. Lee, H. La, H.-J. Kim, J.-I. Yang, H. Jung, ‘Ni catalyst wash-coated on metal monolith with enhanced heat-transfer capability for steam reforming’, Journal of Power Sources, 171 (2007) 499-505] have been used as representative catalyst coating methods.
The impregnation method has problems of an increase in a supporting number to support a certain amount of catalyst because of a low supporting amount of the catalyst and a difficulty in control of dispersion of active metal particles.
In addition, the wash-coating has a problem of a low binding strength between a coating layer and a metal structure because it cannot easily control a thickness of the coating layer and uniformly coat the coating layer, and a large amount of catalyst is needed due to high loss of a coating solution.